Barrett S. Boody, M.D., Spine Surgery

Interactive guide

Understand your lumbar MRI.

Your MRI report is written for radiologists and surgeons. This page is written for you. Click any numbered point on the scan to learn what that structure is, what the report calls it, and why it matters for your back and legs.

Medically reviewed by Barrett S. Boody, M.D. · Updated September 2026
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Go to an emergency department if you have a new change in bowel or bladder control, numbness in the groin or inner thighs, or weakness that is getting worse over hours or days. Those are the signs of cauda equina syndrome, and they need same-day evaluation, not a website.

Interactive labeled lumbar MRI

The click-through viewer needs JavaScript. Every labeled structure is written out further down this page.

How do I read the two views?

Axial means looking down through one slice, and sagittal means from the side. These are T2-weighted images, so anything watery is bright: the fluid around the nerves is the bright stripe, healthy discs look brighter, and worn discs turn dark.

Axial: looking down

A cross-section slice, as if looking down through your body from above. The best view for the canal, the nerves, the facet joints, and the disc at one level.

Sagittal: from the side

A side view showing the whole lumbar spine at once: disc heights, alignment, any slip of one vertebra on the next, and the spinal canal top to bottom.

Bright means water

On T2 images, fluid glows white. Healthy discs look brighter; worn discs turn dark. The fluid around your nerves is the bright stripe running down the canal.

Every labeled structure, explained

Use the viewer above to explore the scan, or read the complete explanations here. The numbers and letters match the labels on the images.

Axial MRI, level L4 to L5

A view looking straight down through one slice of the lower spine.

Intervertebral disc
The cushion between two vertebrae, made of a soft jelly-like center and a tough outer ring. When the center pushes out through the ring, it is called a herniated disc, a very common cause of sciatica. Sciatica and herniated disc.
Back wall of the disc
The annulus fibrosus is the tough outer wall of the disc. The back portion sits right against the nerves, which is why most disc herniations happen here and so often press on a nerve root. Sciatica and herniated disc.
Spinal fluid and nerves
The thecal sac is a fluid-filled sleeve protecting the bundle of nerves (the cauda equina) that runs down your spinal canal. It glows bright white on this MRI. When the canal narrows and squeezes this sac, it causes many symptoms of spinal stenosis. Lumbar spinal stenosis.
Exiting nerve roots
These are the nerves branching off and leaving the spine through small side openings (foramina) on their way to your legs. Pinching of an exiting nerve root is a classic cause of sciatica: pain, numbness, or weakness down the leg. Sciatica and herniated disc.
Facet joints
The small paired joints at the back of each spinal segment that let you bend and twist. Like any joint, they can develop arthritis, a common source of back pain, and when they enlarge they help narrow the spinal canal. Lumbar spinal stenosis.
Ligamentum flavum
A ligament lining the back of the spinal canal. As it thickens with age, it takes up space inside the canal and squeezes the nerves. It is one of the main causes of spinal stenosis. Lumbar spinal stenosis.
Paraspinal muscles
The strong muscles running alongside your spine that support your posture and movement. Protecting these muscles is a key goal of minimally invasive surgery, which works between them instead of cutting through them. Minimally invasive spine surgery.

Sagittal MRI, lumbosacral spine

A side view of the spine, from L1 down through the sacrum.

Vertebral bodies (L1 to L5)
The stacked blocks of bone that form the spinal column and carry most of your weight. They are labeled L1 through L5 from top to bottom. When one slips forward over another, it is called spondylolisthesis. Spondylolisthesis.
Intervertebral discs
The cushions between the vertebral bodies. From the side you can see their height and whether any are worn or bulging. On this T2 MRI, healthy discs hold water and look brighter; degenerated discs look dark. Conditions of the low back.
Sacrum (S1)
The triangular bone just below L5 that joins the spine to the pelvis. The lowest disc, L5 to S1, is one of the most common sites of a herniated disc and sciatica. Sciatica and herniated disc.
Coccyx
The tailbone: the small fused segments at the very bottom of the spine, below the sacrum.
Conus medullaris
The tapered lower end of the spinal cord itself, usually finishing around the L1 to L2 level. Below this point, the spinal canal carries a bundle of nerve roots rather than the cord.
Cauda equina
Latin for horse's tail. Below the spinal cord, the nerve roots fan out and float in spinal fluid on their way to the legs, bladder, and bowel. Sudden compression here (cauda equina syndrome) is a rare emergency. Seek care or call 911 for new severe weakness or loss of bladder or bowel control. Lumbar spinal stenosis.
Thecal sac
The fluid-filled sac (dural sac) that surrounds and protects the cauda equina nerves. It looks bright on this T2 MRI. Narrowing that pinches this sac is what produces many spinal stenosis symptoms. Lumbar spinal stenosis.
Ligamentum flavum
A ligament between the laminae at the back of the spinal canal (highlighted green). When it thickens with age it bulges inward and squeezes the nerves, a leading cause of spinal stenosis. Lumbar spinal stenosis.
Anterior longitudinal ligament
A strong ligament (highlighted blue) running down the front of the vertebral bodies. It stabilizes the spine and limits backward bending.
Posterior longitudinal ligament
A ligament (highlighted orange) running along the back of the vertebral bodies, just in front of the spinal canal. It helps hold the discs in place. Sciatica and herniated disc.

What do the words on my report mean?

Most lumbar reports use a small vocabulary. Each term describes where something is pressing and how much room is left. None of them, by itself, says whether you need treatment.
Common lumbar MRI report terms and their plain-English meaning
Report saysIn plain English
Desiccation, degenerative discThe disc has lost water and looks dark. Extremely common with age and, on its own, not a diagnosis.
Bulge, protrusion, extrusionThree degrees of disc material moving beyond its edge: broad and even, focal, and pushed out with a narrow neck. A sequestered fragment has broken free. Large fragments often shrink over months.
Annular fissureA tear in the tough outer ring of the disc, sometimes seen as a small bright spot at the back of the disc. It can be a source of back pain and is a weak point for a herniation.
Ligamentum flavum hypertrophyThe ligament at the back of the canal has thickened and takes up space. A main cause of spinal stenosis.
Facet arthropathy, hypertrophyArthritis and enlargement of the small paired joints at the back of the segment. A source of back pain, and enlarged joints help narrow the canal and the nerve exits.
Neural foraminal narrowingThe side doorway for one nerve root is tighter than normal. This is the finding behind many cases of sciatica.
Central canal stenosisThe main channel for the nerves has narrowed. Graded mild, moderate, or severe. The classic symptom is heaviness or aching in the legs with standing or walking that eases when you sit.
Spondylolisthesis, anterolisthesisOne vertebra has slipped forward on the one below. Graded by how far. Bending X-rays show whether the slip moves.
Modic changesChanges in the bone on either side of a worn disc. Type 1 reflects inflammation and is sometimes linked to back pain; type 2 is fatty change and is usually quiet.

Which finding explains the leg pain?

Each lumbar level has two nerve roots that can be caught: the traversing root passing through on its way down, and the exiting root leaving through the side doorway. A typical disc herniation at L4 to L5 catches the traversing L5 root. A far lateral herniation at the same level catches the exiting L4 root instead, and the pain follows a different path.

That is why the report and the exam have to agree. The nerve root that explains the symptoms is the one that matters, and the location of the herniation on the axial image, central, at the side, or out in the foramen, tells the surgeon which root it is reaching and which operation would free it. Dr. Boody has published on the far lateral approach for microdiscectomy in Clinical Spine Surgery, the small-incision operation used when the fragment sits outside the canal.

The same logic runs the other way. Leg heaviness that comes on with walking and eases with sitting points to central canal narrowing pinching the whole bundle, the picture of lumbar spinal stenosis. Pain down the back of one leg to the foot points to a single root, most often at L5 to S1, the picture of sciatica from a herniated disc. The scan shows the anatomy; your symptoms say which finding is live.

How Dr. Boody uses it

The target on the MRI decides how small the operation can be.

Many adults with no symptoms have dark discs, bulges, and narrowing on a lumbar MRI. So the first question Dr. Boody asks of a report is not what it lists but which finding explains the leg or the back in front of him. A level that matches the exam is a target. A level that does not is noted and left alone.

When surgery is the right step, the target on the scan sets the plan. A fragment out in the foramen can be reached by far lateral microdiscectomy; a narrowed canal by a small-incision decompression; a level that also slips by a minimally invasive fusion. He has published on the far lateral approach and on whether symptomatic facet cysts need a fusion when the level is stable, both in Clinical Spine Surgery.

In clinic he pulls up your images, not just the report, marks the target on the screen, and tells you whether a fusion is part of the plan and why. That is the same walkthrough his fellows get before a case.

Research and teaching

Questions patients ask

Lumbar MRI, answered.

What is the difference between the sagittal and axial views?
Sagittal is a side view that shows all five lumbar discs, the alignment of the vertebrae, and the canal from top to bottom in one picture. Axial is a cross-section through one level, looking down, and it shows the disc, the fluid-filled sac of nerves, the roots leaving at the sides, and the facet joints. The sagittal view finds the level; the axial view shows what is pressing on what.
Why are some of my discs bright and others dark?
These are T2-weighted images, on which water is bright. A healthy disc holds water and looks bright in the middle. A worn disc has dried out and looks dark, which the report may call desiccation or degenerative disc disease. Dark discs are extremely common with age, appear on the scans of many people with no pain, and are not a diagnosis on their own.
What is the difference between a bulge, a protrusion, and an extrusion?
They are three degrees of disc material moving beyond the disc's normal edge. A bulge is broad and even. A protrusion is focal, a push in one spot. An extrusion has pushed out further, through the outer ring, often with a narrow neck, and a sequestered fragment has broken free entirely. Larger fragments are more likely to press on a nerve, but they also tend to shrink over months as the body resorbs them, which is one reason most herniations settle without surgery.
What does “foraminal narrowing” mean?
The foramen is the small side doorway where a nerve root leaves the spine on its way to the leg. Foraminal narrowing means that doorway is tighter than normal, from a bulging disc, a bone spur, an enlarged facet joint, or a loss of disc height. When it pinches the exiting root it causes pain, tingling, or weakness down the leg in that root's pattern, which is what most people mean by sciatica.
What is “ligamentum flavum hypertrophy”?
The ligamentum flavum lines the back of the spinal canal. With age it thickens and buckles inward, taking up space that the nerves need. Together with enlarged facet joints and a bulging disc it is one of the main causes of lumbar spinal stenosis. The classic symptom is heaviness, aching, or numbness in the legs that comes on with standing or walking and eases when you sit or lean forward.
What is the cauda equina, and when is it an emergency?
The spinal cord ends around L1 to L2. Below that, the canal carries a bundle of nerve roots called the cauda equina, Latin for horse's tail, on their way to the legs, bladder, and bowel. Sudden severe compression of that bundle is cauda equina syndrome. A new change in bowel or bladder control, numbness in the groin or inner thighs, or weakness getting worse over hours or days means go to an emergency department now.
Does a herniated disc on my MRI mean I need surgery?
No. Most lumbar disc herniations improve within weeks to a few months with activity adjustment, medication, physical therapy, and sometimes an injection, and the fragment often shrinks on its own. Surgery is discussed for leg pain that has not settled after that care, for weakness that is not recovering, or for cauda equina symptoms. When it is needed, it is usually a small-incision operation to free the nerve.
Should I bring my MRI images or just the report?
Both, and the images matter more. Bring the disc from the imaging center or your portal access to the images. Dr. Boody reads the pictures, marks the target on the screen, and explains what each finding means for you. That walkthrough is the same one his fellows get before a case.

Next step

Want yours explained, image by image?

Walking patients through their own MRI is part of every consultation. Bring the images and the report. You will leave knowing which finding is live, and whether an operation belongs in the plan. Offices in Carmel, Fishers, Greenfield, and Muncie.

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